In the last article we reviewed the features of python magic methods, __new__() and __init__(). We will look into a real example today about how to use the magic methods to simplify code in Python, with example of URL matching in Pyramid framework.
Python magic methods (1) continued: Use descriptor, __new__ and metaclass to simplify URL Matching
Assume we have code like below. The method get_route_param will get the parameter from current request object in Pyramid by the name.
class TitlesContext(RootContext):
def get_title(self):
title = self._db.query(Title).get(self.get_route_param('title_id'))
if not title:
raise HTTPNotFound()
return title
def get_route_param(self, name):
"""
Return the route parameter with the given ``name`` or raise a 404 if it
is False-y.
:rtype: unicode
:raise: :exc:`pyramid.httpexceptions.HTTPNotFound`
"""
value = self._request.matchdict[name].strip()
if not value:
raise HTTPNotFound()
return value
In one pyramid application, for each Model-View-Controller, there could be multiple context object to query the database objects, thus the method get_route_param could exist in many places.
To improve, we can refactor the code by removing the get_route_param method by creating a new class, UrlMatch:
class TitlesContext(RootContext):
title_id = UrlMatch()
def get_title(self):
title = self._db.query(Title).get(self.title_id)
if not title:
raise HTTPNotFound()
return title
class UrlMatch(object):
"""
Descriptor that will retrieve variable from the current requests matchdict.
This should be used on a context class to allow retrieving matchdict variables
from the context. When the context class uses :class:`UrlMatchMeta` as it's metaclass
then ``UrlMatch`` will use the class attribute name as the key to extract from
the matchdict. If the parent class of ``UrlMatch`` does not use `UrlMatchMeta`
as its metaclass then the ``name`` argument must be given to ``UrlMatch``.
Example with metaclass::
class MyContext(object):
__metaclass__ = UrlMatchMeta
def __init__(self, request):
self._request = request
my_var = UrlMatch()
"""
def __init__(self, name=None):
self.name = name
def __get__(self, obj, type=None):
if not obj:
# Accessed via class, so return the descriptor
return self
if not self.name:
raise ValueError(
'UrlMatch property does not have a name to extract from matchdict. ' +
'Use viewutils.UrlMatchMeta as your metaclass or pass a name to UrlMatch().'
)
try:
return obj._request.matchdict[self.name]
except KeyError:
# Turn the KeyError into AttributeError because this will be accessed as
# an attribute
raise AttributeError("Matchdict does not have key '{}'".format(self.name))
class UrlMatchMeta(abc.ABCMeta):
"""
Metaclass that provides support for the :class:`UrlMatch` descriptor.
This metaclass will set the ``name`` attribute of each ``UrlMatch`` class attribute
to the name of the class attribute.
"""
def __new__(cls, classname, bases, classDict):
new_class = super(UrlMatchMeta, cls).__new__(cls, classname, bases, classDict)
cls._set_url_match_names(new_class)
return new_class
@staticmethod
def _set_url_match_names(new_class):
for key, attr in vars(new_class).items():
if isinstance(attr, UrlMatch):
attr.name = key
In this example, we used descriptor, metaclass __new__() to create this new class to simplify the code.
So how will this work?
-
__metaclass__ is used to redefine how a class is constructed. In UrlMatchMeta, __new__() is called to add the key in MyContext’s attributes (in this case, “title_id”) to the name of UrlMatch() object. Note __new__() is called automatically when calling the class name, even before __init__(). By using metaclass, we have created a UrlMatch object called title_id in TitlesContext and dynamically added title_id as its name.
-
__get__() descriptor is used to retrieve the attribute from request context. Python descriptor says if we call obj.d, it will look for d in obj’s dictionary, and if d defines __get__(), it will call d.__get__(obj). In our case, when we call self.title_id (self is the TitlesContext), it finds __get__() method, so it will actually call title_id.__get__(TitlesContext). Since title_id is UrlMatch, it will return obj._request.matchdict[self.name], which is TitlesContext._request.matchdict[“title_id”]. As such, we complete the same way as to look for title_id parameter in request (which is the same as get_route_param from the old way).
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